Providing an effective and long-term annular seal for fluid flow devices is a continuing problem in the fluid flow industry. In order to effectively seal an annular area adjacent to a valve, stem, rod or similar sealing application, in a fluid flow device, the force or load, F applied to the seal component, must equal the pressure, P under which the seal element is exposed, multiplied by the annular area, A to be sealed (F=PA). If the force or load, F applied to the seal component does not equal the pressure, P multiplied by the annular area, A to be sealed, the seal element can not effectively seal the annular area. The known annular seals for these types of above-identified fluid flow devices have been designed on the principal of either compression type or self-energizing lip type seals, or some combination thereof. Compression type seals, also known as squeeze type seals, typically incorporate a resilient material which is exposed to a compressive stress, wherein the compressive stress supplies the energy needed to create the initial seal. This type of seal may, for example, be inserted into a gland formed by two rigid sealing surfaces, wherein the gland has a seal receiving portion which has a slightly smaller outside diameter as compared to the outside diameter of the seal. This type of seal/gland arrangement applies compressive stresses to the seal, when the two mating surfaces contact the seal. As pressure is applied to the system, the initial compressive forces are augmented by the fluid flow pressure, or system pressure.
Self-energizing lip type seals generally achieve initial sealing by placing the seal lips under bending stress and exposure to the system pressure, wherein the lips are forced tightly against the gland surfaces.
In view of current Environmental Protection Agency guidelines for allowable leakage and fugitive emissions requirements for fluid flow devices, conventional lip and compression type seals often do not comport with acceptable standards. Additionally, depending upon the particular design, lip and compression type seals are prone to premature wear and failure, requiring frequent and costly maintenance. In dynamic applications, where at least one of the orifice or gland surfaces is in motion, wear rates typically increase in response to the increase in load or force applied to the seal element. Energized seals may be especially prone to premature wear and failure in this type of application. Additionally in dynamic applications involving high system pressures, once the system pressure is removed, the high pressure forces which were exerted on the seal often damage or destroy the initial interference of the seal, which is responsible for providing the initial stresses required to seal the annular area. When the initial interference of the seal becomes damaged or destroyed, the seal may cease to properly function.
In static applications, where neither gland surface is in motion, typically, tight, compressive seals have been employed. This type of static seal, includes squeeze type seals which rely upon the compressive stress created when the gland surfaces contact the seal, to provide the initial sealing forces. These types of seals include, for example, washers, gaskets, packings, o-rings and the like. Squeeze type seals due to their construction and composition, have a limited usefulness. Further, over time, static seals may experience stress relaxation which adversely affects the resilience of the seal.
Moreover, known seal materials which are solely constructed of rubber and other elastomeric materials are generally not conducive to long-term exposure to caustic, corrosive and harsh chemicals and oils, and may break-down and/or fail in a relatively short period of time. Encapsulation and partial encapsulation of rubber and elastomeric seal devices to create a protective coating to reduce the amount of exposure of the seal to harmful substances, is expensive and greatly restricts the type of seal available, due to molding process capabilities and other restrictions. Partial encapsulation of the rubber or elastomeric seal device is less expensive, but may not avoid exposure to and break-down of the seal device. Additionally, both the full and partially encapsulated rubber or elastomeric seal devices require the use of separate back up or anti-extrusion components at higher pressures, causing additional expense and assembly. An additional problem exists with the above discussed types of seals, in that these seals do not automatically adjust to wear, misalignment, temperature and pressure differentials and the changing characteristics of the mating surfaces for which they are used.
Fluid flow devices, such as valves, pumps, stuffing boxes, hydraulic devices, flow meter orifices and the like, are typically constructed of metal, and therefore typically have orifice or gland mating surfaces which expand and contract in response to temperature and pressure differentials. Often, seal components which fit snugly within the seal receiving portion of the gland may contract with the change in temperature such that the outer diameter surface of the seal breaks contact with the inner surface of the seal receiving portion of the gland, resulting in a leak path between the seal and the gland. It would be highly advantageous to create an annular seal, which included the benefits of elastomeric materials, and which also maintained its original dimensions during temperature and pressure differentials. Seal materials such as polytetrafluoroethylene, polyvinylchloride, polyethylene and other polymer materials are highly resistant to caustic, corrosive and harsh chemicals and oils, but have little or no elastomeric properties and therefore do not adapt well to the changing temperature and pressure characteristics of the mating surfaces. Additionally, it would be advantageous to achieve an annular seal adaptable to variable surface characteristics and variable seal performance characteristics.
In view of the lack of an effective annular seal for fluid flow devices such as valves, pumps, stuffing boxes, hydraulic devices, in-line flow meters and the like, which are thermally stable, resistant to pressure and stress relaxation, and which can withstand exposure to caustic, corrosive and harsh chemicals and oils over long periods of time and can expand and contract with the changing characteristics of the mating surfaces, a need for the present invention exists.